1. Field of the Invention
The present invention relates to a liquid ejection head and an image forming apparatus comprising same, and more particularly to a liquid ejection head and an image forming apparatus comprising same that can achieve a high-density arrangement of ejection ports ejecting a liquid while also permitting ejection of high-viscosity liquid.
2. Description of the Related Art
Conventionally, as an image forming apparatus, an inkjet printer (inkjet recording apparatus) is known which comprises an inkjet head (liquid ejection head) having an arrangement of a plurality of nozzles (ejection ports) and which records images on a recording medium by ejecting ink from the nozzles toward the recording medium while causing the inkjet head and the recording medium to move relatively to each other.
In the inkjet printer of this kind, ink is supplied to pressure chambers from an ink tank via an ink supply channel, and piezoelectric elements are driven by supplying electrical signals corresponding to the image data to the piezoelectric elements. Thereby, the diaphragm constituting a portion of each pressure chamber is deformed, the volume of the pressure chamber is deformed, and the ink inside the pressure chamber is ejected from a nozzle in the form of a droplet.
In the inkjet recording printer, one image can be formed on a recording by combining dots formed by ink ejected from the nozzles. In recent years, it has become desirable to form images of high-quality on a par with photographic prints, according to inkjet printers. It has been considered that high image quality can be achieved by reducing the size of the ink droplets ejected from the nozzles by reducing the diameter of the nozzles, while also increasing the number of pixels per image by arranging the nozzles at high density. As a method of increasing the density of the nozzle arrangement, conventionally, it has been proposed that nozzles be arranged in a two-dimensional matrix array.
For example, it is known in which a plurality of nozzles are arranged in the form of a lattice comprising a plurality of rows inclined at an uniform angle with respect to the main scanning direction of the head and a plurality of columns perpendicular to the main scanning direction of the head, and that the planar shape of the diaphragm which constitutes one surface of the pressure chambers provided respectively corresponding to each nozzle is formed to an approximately square shape or diamond shape. Thereby, it is possible to increase the ejection efficiency of the pressure chambers, and to arrange the nozzles at high density (see Japanese Patent Application Publication No. 2001-334661, for example).
Furthermore, a technology is also known in which pressure chambers provided in a cavity plate are formed in an approximate diamond shape, an ink supply port being formed in one of the acute corner sections of each pressure chamber, and an ink ejection nozzle being formed in the other acute corner section of same. Thereby, since a plurality of ink pressure chambers are arranged corresponding to a plurality of nozzles without increasing the dimensions of the cavity plate, high density of the nozzles can be achieved (see Japanese Patent Application Publication No. 2002-166543, for example).
However, when the density of the nozzles is increased by the inkjet head having a composition described in the aforementioned references, there are problems such as described below. Therefore, it is difficult to achieve efficient ink ejection if the nozzles are arranged at high density in the composition of this kind, in practice.
More specifically, in a composition which ejects ink from one nozzle of an inkjet head as disclosed in the aforementioned references, the common ink flow channel, supply channel, pressure chamber and nozzle are all disposed on the same one side of the diaphragm which forms one surface of the pressure chamber, and the piezoelectric actuator is disposed on the opposite side thereof in the diaphragm.
For example, if the density of the nozzles is increased in a composition of this kind, then the size of the common flow channel gradually decreases as the density thereof rises. Therefore, when the ink is ejected by driving a plurality of nozzles at high frequency, the ink supply to the pressure chambers may not be sufficient. In this case, if the common flow channel is increased in size in order to obtain a smooth supply of ink, then the actual ejection operation may become difficult to perform, due to the increased distance between the pressure chamber and the nozzle. Consequently, there is a problem in which the ejection frequency cannot be raised due to structural limitations on the size of the common flow channel.
In addition, if the ink of low viscosity is used, then the ink landing on the recording medium permeates rapidly into the recording medium, thereby giving rise to bleeding, which leads to degraded image quality. Therefore, it is desirable to use high-viscosity ink from the viewpoint of preventing bleeding of this kind. However, when a high-viscosity ink is used in a head having nozzles arranged at high density as described above, there is a problem in which the ink is supplied more slowly to the pressure chambers due to the reasons described above, and hence the ejection frequency becomes even lower.